diff --git a/doc/functions.sgm b/doc/functions.sgm
index 2cca23d..232b755 100644
--- a/doc/functions.sgm
+++ b/doc/functions.sgm
@@ -167,7 +167,7 @@
             <para>
               returns a boolean value that signifies whether the points
               <parameter>p1</parameter> and <parameter>p2</parameter>
-              lie within distance <parameter>radius</parameter> of each other, i.e.
+              lie within distance <parameter>radius</parameter> (in radians) of each other, i.e.
               it computes the boolean expression <literal>p1 &lt;-> p2 &lt;= radius</literal>.
               On PostgreSQL 12 and later, the function has <literal>GiST</literal>
               support and the PostgreSQL optimizer will transform it to either